The planar waveguide as a laser medium has the advantages of confining the laser radiation closely within the gain region, allowing extremely efficient cooling of the gain region, and providing a laser power which may scale in proportion to the area of top and bottom surfaces of the planar waveguide. With reference to FIG. 1, key terminology is now defined for a planar waveguide gain region 1. The section of planar waveguide in this case has a core 2 of active material of core height (or “thickness”) a, width w and length L. The ratio w/a is normally considerably greater than unity for the term “planar waveguide” to be used and for the resonator configuration described here to be of interest. The core 2 is sandwiched between an upper cladding layer 3 and a lower cladding layer 4. The cladding layers 3, 4 provide the effect of guiding laser light substantially through the core region. FIG. 1 also shows common terminology used in describing the propagation of light in a planar waveguide and followed later in this document. The resonator axis, which is also the direction of propagation of light in the planar waveguide, is denoted by the direction Z. The direction normal to the laser resonator axis and propagation of light is referred to as the lateral direction, X, when it lies in the plane of the planar waveguide. The direction normal to the planar waveguide is referred to as the transverse direction, Y.
In long-standing prior art for planar waveguide lasers, also commonly referred to as slab waveguide lasers, the planar waveguide has commonly been of the hollow type with the gain produced by an internal electrical discharge in a laser gas. (Tulip, U.S. Pat. No. 4,719,639, Abramski et al. Applied Physics Letters 54, 1833 (1989), Jackson et al. Applied Physics Letters 54, 1950 (1989)). In these cases, the gas discharge region takes the place of the core region shown in FIG. 1 and metal electrodes take the place of the claddings. The laser resonator for use with such lasers has been commonly referred to as being of a hybrid type, as it combines the properties of free-space and waveguide laser resonators. Such resonators collect light from the extended area within the planar waveguide and emit it into a high quality, near-diffraction-limited laser beam, by coupling it out around the edge of one of the two resonator mirrors.
In the plane which is parallel to the guiding surfaces, known as the lateral axis, the resonator is the type known as the confocal unstable resonator, as described for example by Siegman in chapters 22 and 23 of his textbook “Lasers” (published by University Science Books, Mill Valley, Calif., 1986). Unstable resonators are widely used to produce a near-diffraction-limited beam from a resonator mode of large volume, usually by coupling light out of the laser around the edge of one end mirror, often abbreviated by the term “edge-coupling”. Exceptionally, the output beam may be coupled through one end mirror with a partially reflective coating, known as a “continuously-coupled unstable resonator” or be coupled through a coating whose reflectivity varies with position across the mirror, often referred to as a “variable reflectivity output mirror” or VRM. A specific and advantageous configuration of unstable resonator is termed “confocal” and provides a substantially collimated output beam. The term “confocal” means that the two curved mirrors forming the resonator are positioned so that their focal points for parallel incident light are at the same point in the resonator.
When applied only in the lateral direction within a planar waveguide, the confocal unstable resonator may be of the positive branch type (Tulip, U.S. Pat. No. 4,719,639, Jackson et al. Applied Physics Letters 54, 1950 (1989)) made up of a concave rear mirror and convex output mirror with a cut-away edge to transmit the output beam. Such a resonator is now known to have a high sensitivity to mirror misalignment and is not usually preferred. Alternatively a negative branch unstable resonator may be used, made up of two concave mirrors (Nishimae et al. U.S. Pat. No. 5,048,048, Hobart et al. U.S. Pat. No. 5,335,242). This has a low sensitivity to mirror misalignment but produces an undesirable focussed beam within the laser gain medium. This can be tolerated in the case of gas lasers operating with a hollow planar waveguide as the high intensity at the focal point does not produce damage or breakdown in gas. Consequently, the negative branch type of unstable resonator is most commonly used in commercially-manufactured gas lasers which use a planar waveguide structure. The confocal negative branch resonator best matches a planar waveguide gain region with parallel lateral boundaries as in FIG. 1. However, useful resonators may be “near-confocal” with an output beam which is somewhat diverging or converging but substantially similar to the precisely confocal resonator.
In the plane that intersects the laser axis normal to the guiding surface, known as the transverse direction, the resonator mirrors may be spaced at various distances away from the position where light is no longer guided by the planar waveguide in the transverse direction. Degnan and Hall, in IEEE Journal of Quantum Electronics QE-8, 901 (1973), analysed the problem of the positioning of curved resonator mirrors outside a hollow waveguide and introduced a well-known classification of three beneficial configurations where resonator losses are low (referred to as Cases I, II, and III). Their analysis applies equally to a planar waveguide but in the transverse direction only. In these beneficial cases, light that leaves the waveguide with a transverse profile characteristic of the fundamental waveguide mode returns to the waveguide with a substantially unchanged profile after reflection from the resonator mirror. Typically more than 98%, and in some cases nearly 100%, of the light leaving the waveguide is correctly coupled back into the fundamental mode of the waveguide. The residual fraction, typically less than 2%, is coupled to higher order waveguide modes or is otherwise lost. By ensuring that each of the external mirrors is configured in one of the three beneficial cases, the resonator may have a low value of round-trip loss for the fundamental mode associated with the interchange between guided wave propagation and free-space propagation. This unwanted round trip loss may be controlled to be considerably lower in value than the resonator output coupling fraction to ensure efficient laser operation. The unwanted resonator loss may be held certainly below 4% per round trip and usually in the region of 2%. The term “low-loss waveguide resonator” is understood to mean a resonator that is configured to achieve these indicative low values of round trip loss. Resonators where mirrors are positioned not in the beneficial cases have significantly higher loss and reduced laser efficiency.
The beneficial Case I occurs when the mirror is plane and near to touching the end of the waveguide, and “near-Case I” corresponds to a mirror which is curved with a long radius of curvature placed close to the end of the waveguide. A Case II configuration occurs when the mirror is placed at a distance from the waveguide sufficient for it to be in the far-field diffraction pattern of the fundamental waveguide mode and the concave radius of curvature of the mirror is equal to its distance from the waveguide. The Case III configuration has a mirror chosen to be of concave radius of curvature close to R=0.66 a2/λ and positioned at a distance R/2 from the end of the waveguide, where a is the core thickness of the planar waveguide core (in the transverse direction), and λ is the wavelength of emission of the laser.
In the class known as a dual-Case I waveguide resonator, both laser mirrors are in close proximity to the ends of the planar waveguide. For the hollow planar waveguide gas laser, the waveguide transmission loss increases rapidly with transverse mode order and acts to limit the laser oscillation to the lowest order mode of propagation of the waveguide. Consequently there is generally no need to provide another transverse mode selection means and placement of the mirrors close to the ends of the waveguide in the dual-Case I configuration is satisfactory for gas lasers. However, a small additional space between each mirror and the corresponding end of the waveguide can be beneficial in promoting additional mode selection and ensure the lowest resonator loss (as described by Hobart et al. U.S. Pat. No. 5,335,242).
The hybrid waveguide-unstable resonator concept is equally applicable to solid-state planar waveguides using typically a doped solid-state laser material as a core and un-doped material of lower refractive index as upper and lower claddings. Alternatively, the claddings may be an unrelated transparent optical material of appropriate refractive index and physical properties. Optical pumping of the material by for example diode lasers provides excitation of the core material. Alternatively the planar waveguide may be part of a semiconductor laser diode of the broad junction type, pumped by electrical current.
In prior art, a hybrid waveguide resonator of the positive branch type has been described using a planar waveguide made from yttrium aluminium garnet (YAG) with neodymium doping of the core (Nd:YAG) (A A Chesworth, PhD dissertation, Heriot-Watt University, 1998; Baker et al. Optics Communications, 191, 125 (2001), Lee et al. Optics Letters, 27, 524 (2002)). This laser used the positive branch unstable resonator configuration to avoid the high intensity focussed beam of the otherwise more desirable negative branch configuration. As a consequence of using the positive branch resonator, the mirror radii of curvature are relatively large and this laser is adversely affected by the additional variations of focal power generated within the resonator by, for example, non-uniform pumping of the planar waveguide core in the lateral direction.
When the solid-state planar waveguide has a core thickness and refractive index difference which allows propagation of multiple transverse waveguide modes (a multi-mode waveguide) there is little difference in the propagation loss for each guided mode. It is difficult to achieve a laser output in predominantly the lowest order transverse mode. This is a major difference in behaviour between hollow waveguide gas lasers described above and the solid-state waveguide lasers. The above cited Nd:YAG planar waveguide laser has the resonator mirrors placed very close to the end-faces of the active waveguide section, forming a good approximation to the low loss, dual-Case I waveguide resonator in the transverse direction. It operates most efficiently whilst emitting a non-diffraction-limited transverse beam made up of typically six transverse waveguide modes. In the present state-of-the-art using the positive branch type of hybrid waveguide-unstable resonator with mirrors in the dual-case I position, it is difficult to provide a mode selection means to avoid this effect. However, most applications of lasers require the best available beam quality and operation in multiple transverse modes is undesirable.
In an alternative form of this prior art (Meissner, U.S. Pat. No. 6,160,824), some of the difficulties of a multi-mode waveguide are overcome by using a planar waveguide core that is very thin, supporting only the guiding of a single transverse mode. As no transverse mode selection method is then needed, the end-faces of the solid-state waveguide may be directly ground and polished to produce the curved end-mirrors for a positive branch type of hybrid resonator. However in this configuration, the use of the preferred negative branch resonator type is still precluded by the presence of an unavoidable, intense focal point within the solid material of the waveguide core. Also in this configuration, the maximum pump power that may be applied to the active region is limited by the use of a single-mode waveguide.
It is an aim of the present invention to provide a laser resonator which avoids or minimises one or more of the foregoing disadvantages.